Add initial toc component and handling in preview

[Komediruzecki]

Add linkable headers custom link
Add event for firing focus heading

Tested in dev desktop app (cloud space)
Tested in webapp (cloud space)

Workflow:

1. User creates document with headings (h1-h6 - depest level 6, can be changed later)
2. User adds toc with shortcode [[toc]]
3. User clicks on markdown preview links and navigates to editor heading location
4. User disables sync and clicks on links (only focuses in editor, markdown preview stays where it is)

Example: